1. Field of the Invention
The invention relates to a fuel cell system that generates electric power using fuel gas supplied to an anode and air supplied to a cathode and to a method for controlling such a fuel cell system.
2. Description of the Related Art
Japanese Patent Application Publication No. 09-312167 (JP-A-09-312167) describes a fuel cell system that performs power generation with fuel gas being retained in an anode-side gas passage (will be referred to as “anode-dead-end type fuel cell system”). In this fuel cell system, a fuel supply valve is provided in the upstream side of the anode-side gas passage and a gas-discharge valve is provided in the downstream side of the anode-gas passage.
The amount of fuel gas required to be supplied to the fuel cell unit changes depending upon the value of the power output required of the fuel cell unit, and so on. Therefore, the fuel supply valve is controlled such that an appropriate amount of fuel gas is supplied to the fuel cell unit. Further, in anode-dead-end type fuel cell systems, impurities, such as nitrogen and water, accumulate in the anode-side gas passage as the operation of the fuel cell unit continues. In anode-dead-end type fuel cell systems, therefore, the gas-discharge valve is opened at a given timing or intermittently so as to discharge the impurities accumulating in the anode-side gas passage to the outside via the downstream end of the anode.
In the above-described fuel cell system, valve mechanisms are provided in the upstream side and the downstream side of the anode, respectively, and controlled so as to maintain a good power generation state of the fuel cell unit. However, in view of improving the system reliability and ensuring a high robustness, it is desirable to make the system configuration as simple as possible.